The present invention relates to a sensor for detecting a change in a physical property.
In an automobile, a magnetic sensor is used as a steering wheel angle sensor to detect the rotational angle of a steering wheel.
Referring to FIG. 3, a magnetic sensor 1 includes four magnetoresistance elements 31 to 34, which form a Wheatstone bridge. A voltage difference is produced between node N1 of the magnetoresistance elements 31 and 32 and node N2 of the magnetoresistance elements 33 and 34. If the voltage difference between nodes N1 and N2 (difference between median potentials E1 and E2 of the bridge, or offset voltage E12) is closer to 0 V in a state in which the magnetic field of the magnetic sensor 1 extends in a predetermined direction, the magnetic sensor 1 would have a higher capacity. More specifically, when R1, R2, R3, and R4 respectively represent the electrical resistances of the magnetoresistance elements 31, 32, 33, and 34, it is preferred that the equation of R1×R4=R2×R3 be satisfied.
For the offset voltage E12 to be closer to 0 V, that is, for the above equation to be satisfied, the magnetoresistance elements 31 to 34 of the magnetic sensor 1 has predetermined patterns so that the electrical resistances R1 to R4 become equal to one another. Accordingly, the magnetoresistance elements 31 to 34 are formed by etching a magnetoresistance element formation film in accordance with the predetermined patterns. However, the accuracy in film formation and etching affects, and causes differences, in the electrical resistances R1 to R4 of the magnetoresistance elements 31 to 34. Furthermore, it is difficult for the above equation to be satisfied just by improving these accuracies.
Accordingly, a trimming process for finely adjusting the electrical resistances R1 to R4 by trimming parts of the magnetoresistance elements 31 to 34 has been proposed in the prior art (refer to, for example, Japanese Laid-Open Patent Publication No. 05-034224).
However, even when performing the trimming process to satisfy the above relationship, wear starts to progress at different rates in the magnetoresistance elements 31 to 34 immediately after the trimming process. Thus, as time elapses, the Wheatstone bridge becomes imbalanced. As a result, the offset voltage E12 changes from 0 V soon after the trimming process to a level that affects the capacity of the magnetic sensor 1. Such a change cannot be ignored.
Accordingly, prior to the trimming process, the magnetoresistance elements 31 to 34 may be irradiated by a laser beam and heated to physically stabilize the magnetoresistance elements 31 to 34. However, when a process using a laser beam (heat treatment process) is performed in addition to the trimming process, a device for laser beam emission becomes necessary. This raises the manufacturing cost of the magnetic sensor 1.